Introduction to special issue

Microbial Biology: Tracing the Mother of All Cells

See allHide authors and affiliations

Science  02 May 1997:
Vol. 276, Issue 5313, pp. 700
DOI: 10.1126/science.276.5313.700

When evolutionist Carl Woese unveiled the once-hidden land of the Archaea in 1977, he not only revolutionized microbial classification but also ushered in a new era in one of biology's grandest, if most problematic, pursuits: understanding the origins of life. Researchers had been exploring this question with test-tube experiments on organic molecules for years, but those studying living microbes had little to add to the debate. By sketching the first complete family tree of the microbes, however, Woese allowed researchers to search for organisms near the tree's base—close to the ancestral one-celled form. The features of those organisms may open a window on the era that preceded cells, when free-floating molecules first developed the ability to replicate and evolve.

“Before Woese, microbiologists simply wouldn't touch the question of what the last common ancestor looked like,” says Alan Weiner, a molecular biologist at Yale University. Now hardly a month goes by without a fresh insight or meeting on the subject, from an April Nobel forum in Stockholm to an American Chemical Society symposium in June. The results so far suggest that the first organisms were probably not bacteria but archaea. And instead of evolving in a mild soup of organic molecules—as was first suggested by Charles Darwin—these organisms may have been born in what humans would consider marginal environments, such as boiling, sulfurous pools or hot, mineral-laden, deep-sea volcanic vents. “For the first time, we can go into the deep, totally unobservable past to test our predictions about the early evolution of life,” says Günter Wächtershäuser, an evolutionary biologist at Germany's University of Regensburg.

The leading theory has long held that life began when lightning charged a warm soup of organic molecules with energy and caused them to begin replicating; variations include the possibility of a cold ocean as the cradle of life, an idea now advocated by longtime origin-of-life expert biochemist Stanley Miller of the University of California, San Diego.

But to many researchers, Woese's tree points to a different conclusion. Originally based on RNA and now confirmed by other genetic sequences (see main text), the tree separates all living organisms into three domains: Archaea (microbes that often inhabit extreme environments), Bacteria, and Eukarya (all multicellular animals and plants). By comparing sequences from the three domains, Woese could trace where various groups of organisms had branched off. And by following the branches back down the tree, Woese, and later other scientists, could identify organisms close to the shared common ancestor for the archaea and bacteria (see diagram in “Microbiology's Scarred Revolutionary”). Most of these turn out to be thermophiles or hyperthermophiles—Archaea and Bacteria that thrive at 80°C or higher.

That suggests that the last common ancestor was a hyperthermophile, says John Baross, an evolutionary microbiologist at the University of Washington, Seattle. Extrapolating back from the first fossil evidence of microbes at 3.8 billion years ago, he and others estimate that this organism lived about 4.3 billion years ago. Going even further back, researchers such as Everett Schock of Washington University in St. Louis conjecture that life began in an anaerobic, “nasty and hot” environment much like Yellowstone's sulfurous hot springs or a deep-sea vent. In addition, many of the most ancient organisms on the Woese tree are autotrophs, with metabolism based not on organic compounds but on inorganic material, such as carbon dioxide or hydrogen sulfide. Some scientists argue that the first cell was an autotroph, too.

That scenario matches many scientists' view of the early Earth. For example, Stanford University geochemist Norman Sleep has proposed that the ancient atmosphere contained high, greenhouse levels of carbon dioxide and was constantly being bombarded by meteorites and asteroids; some of these were as “big as Mount Everest” and sparked enough heat to boil off a 3-kilometer-deep ocean, says Baross. “That would have caused the death of any organism,” he adds, “so where are the safe places on early Earth? In deep-sea volcanic vents, or subsurface sea-floor cracks”—just the places the primitive hyperthermophiles love today.

Trying to divine the branching pattern of the tree—and hence which organisms lie near the base of it—from the genetic sequences of living microbes has pitfalls, however. Chief among them is that early in history, “life was not chaste,” as Antonio Lazcano, an evolutionary biologist at the National University of Mexico in Mexico City, puts it. Clues from living bacteria suggest that, like characters in a John Updike novel, different species freely swapped genetic material or simply picked up exogenous free-floating DNA, leaving a confusing trail.

Still, Wächtershäuser has recently argued that some traits are not easily swapped—notably those related to temperature. He theorizes that it is impossible for “a hot organism to exchange genes with a cold organism, and vice versa,” because the proteins regulating cellular functions are tuned to work at a specific temperature. The implication is that the first cell did indeed like it hot. What's more, because it is easier to activate proteins at warm-to-hot temperatures, it makes sense that life evolved in a heated soup, then adjusted to lower temperatures as Earth cooled, he says.

But even if the first cells inhabited hot springs, the molecules that preceded them could have originated in a milder, “Club Med” environment, say Miller and others. They point out that even more than 4 billion years ago, the ancestral cell was already sophisticated, with a genome and the ability to replicate. “The hyperthermophiles may be ancestral to later life, but they are hardly primitive,” says Miller. “They are as complicated as we are.” He and Lazcano argue that life could have as easily started in warm, balmy seas—or even cold ones—and later given rise to both hyperthermophiles and lower temperature organisms. A megameteor strike might then have killed off all but the heat-loving organisms, leaving them to give rise to all later ones, says Miller.

Those who favor an archaeal origin of life admit that they need to go back further into the past. Baross argues that hydrothermal vents themselves are windows into the nature of the early Earth—and that the genes of some vent organisms may therefore be living relics. Indeed, Wächtershäuser and Claudia Huber, a chemist at the Technical University of Munich in Germany, took their scenario further back into the past by showing that chemical reactions at vents could lead to the synthesis of key biological molecules (Science, 11 April, p. 245).

Then again, none of these scenarios may be right. Perhaps life evolved elsewhere in the solar system and came blasting into Earth on a bolide. Not even that would knock down the new tree of life, says Norman Pace, an evolutionary microbial biologist at the University of California, Berkeley. “I wouldn't be surprised that if we dredge up some martian creature, we'll find it rooted in Woese's big tree”—making that tree truly universal.

View Abstract

Navigate This Article